Scanning acoustic microscopes may be used to nondestructively inspect bonded wafer pairs in the semiconductor industry, e.g., for air type defects in the interior of the bonded wafer pair, which can occur at internal interfaces. However, inspecting a warped bonded wafer pair can present challenges in such inspections. Some semiconductor products in particular may result in bonded wafer pairs that are prone to warping, such as back-side-illuminated (BSI or BI) sensors, complementary metal-oxide-semiconductor (CMOS) products, and micro-electro-mechanical systems (MEMS) sensors, among others. This may be due to differences in the coefficients of thermal expansion (CTE) of the material layers in the bonded wafer pairs present in these semiconductor products. In the past, bonded wafers were usually level or flat to within about 300 μm, but recently bonded wafers often include warpage of up to about 5-6 mm. As the wafer bonding process is finalized, warpage may be reduced to about 1-2 mm, but nevertheless this warpage can still present issues for inspection using scanning acoustic microscopes and the like.
Before being ultrasonically inspected, a bonded wafer pair may be placed on a wafer chuck to secure the bonded wafer pair during scanning, e.g., at speeds that can reach about 1.5 m/s. The wafer chuck may incorporate a form of vacuum to hold the bonded wafer pair in place, e.g., by including various vacuum rings for supporting and forming a vacuum seal with different sized wafers.
Wafer chucks of the prior art may assume that the bonded wafer pair to be inspected will be sufficiently flat in order to create a proper vacuum seal, which may be needed to hold the bonded wafer pair in place. That is, as long as a sufficient vacuum seal can be formed, minimal warpage (e.g., less than or equal to about 300 μm) may be reduced even further depending on the materials and thicknesses used for the bonded wafer pair. However, many contemporary bonded wafer products (such as those recited above) include too much warpage for the wafer to form a sufficient vacuum seal on a prior art wafer chuck for inspection with a scanning acoustic microscope. To this end, in the prior art, techniques to provide a sufficient seal may include pressing down on the wafer, either by hand or using a mechanical mechanism that can potentially damage a bonded wafer product. Thus, there remains a need for improved wafer chucks, e.g., to support and secure warped bonded wafer pairs for inspection with a scanning acoustic microscope.